Called Zello, the app lets you use your phone as a walkie-talkie or two-way radio as long as you have a network or WiFi connection. Users can join channels and instantly send messages or photos, and the app even works over older 2G networks.

Earlier this week, Zello was ranked No. 4 among social networking apps in the US, as well as No. 38 among all iPhone apps overall. According to app analytics firm Sensor Tower, that was a major jump from just one day ago — on Monday, Zello was ranked No. 25 among social networking apps and No. 389 overall.

Now, Zello is ranked No. 1 overall in the App Store.

This isn't the first time Zello has come in handy. Back in August, Zello saw a significant jump in new installs as Tropical Storm Lane approached Hawaii. And last year, Zello reached No. 1 on the App Store as Hurricane Irma ripped through the Caribbean.

If predictions hold, Hurricane Florence may be the strongest storm ever to make landfall north of Florida. It's important to note that if cell towers are wiped out in the storm and WiFi goes down, Zello will not work. But if you have cell service — even a 2G or 3G connection — the app could help in the event of an emergency.

Here's how it works.

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Zello is free to use and doesn't have ads. To start using the app, though, you'll need to make an account. You can provide as much or as little information as you'd like, including a photo, description, and voice greeting.

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Zello takes a no-frills approach to its interface. By clicking the menu button, you can check your volume level, change your status, view your contacts, and see available channels.

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Zello lets you join existing channels or create your own. To check out some of the available channels, click "Channels" in the menu, then navigate to "Trending Channels."

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You can also search for a specific topic, like Hurricane Florence. When you click on a channel, you'll get an overview of what people are talking about before you join.

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Once you join a channel, you'll hear audio from other members right away, much as if you were listening to a police scanner. You can hear everything posted in the channel. When I listened, I mostly heard people checking to see if the app worked for them as they prepared for the storm.

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Once inside a channel, you can also see a list of users and listen to some of the voice messages people have posted.

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If you don't want to listen to the audio transmissions, you can turn them off by tapping the power button. You'll still be a member of the channel, but the sound will be muted.

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If you want to transmit a message, just press the record button. You can also send a photo by clicking the camera button on the left. Your message will post to the channel, where anyone can listen to it and respond.